1. Field of the Invention
The present invention relates to cables, in particular electrical cables for low-voltage power transmission or for telecommunications, or alternatively for data transmission, as well as mixed power/telecommunications cables, which have self-extinguishing properties and produce a low level of fumes, and to flame-retardant compositions used therein.
2. Description of the Related Art
Self-extinguishing cables are generally produced by extruding over the core of the cable a flame-retardant coating consisting of a polymer composition to which flame-retardant properties have been imparted by the addition of a suitable flame-retardant filler of inorganic type, generally a hydroxide, a hydrated oxide or a hydrated salt of a metal, in particular of magnesium or aluminum. The polymer base generally consists of copolymers of ethylene and ethylenically unsaturated esters, in particular ethylene/vinyl acetate or ethylene/ethyl acrylate copolymers, optionally mixed with polyolefins (see for example patents U.S. Pat. No. 4,673,620 and EP-530,940).
Patent application WO 96/23311 describes a low-voltage, high-current cable in which the inner sheath, the insulating coating and the outer sheath consist of the same black-coloured base material and the insulating layer contains a longitudinal coloured stripe for identification purposes. The use of this material for the various layers would not require the separation of the various components in a recycling process. The base material can, depending on the maximum working temperature of the cable, be a polyethylene with a density of between 0.92 and 0.94 g/cm3 and a Shore D hardness xe2x89xa742, or a thermoplastic elastomer based on polypropylene, for example polypropylene modified with an ethylene/propylene copolymer or a polypropylene-based reactor mixture wherein the elastomeric phase content is greater than 25%. When flame-retardant properties are required, it is no longer possible to use the same material for the various coating layers of the cable, and as a polymer base for the layer containing the flame-retardant filler, the use of ethylene/vinyl acetate copolymers or ultra-low-density polyethylene (ULDPE) and, in particular, ethylene-based copolymers obtained with metallocene catalysts is suggested.
On the basis of the Applicant""s experience, in order to achieve successful results in the flame-resistant tests commonly carried out on self-extinguishing cables, the amount of flame-retardant filler required is high, generally greater than 30% by weight, usually more than 50% by weight, relative to the total weight of the flame-retardant coating. Such a high level of inorganic filler leads to a deterioration in processability and in mechanical properties of the flame-retardant composition, in particular as regards elongation at break and stress at break. Then, the Applicant has found that, in order to obtain a self-extinguishing cable which satisfies the specifications required by the market, it is necessary to have available a polymer base which is capable of incorporating large amounts of flame-retardant filler and, at the same time, of maintaining good mechanical properties, in particular as regards elongation at break and stress at break.
The Applicant has now found that it is possible to produce self-extinguishing cables with high flame resistance and excellent mechanical properties by using as flame-retardant coating a mixture of a flame-retardant inorganic filler and a polymer base comprising a heterophase copolymer having an elastomeric phase based on ethylene copolymerized with an xcex1-olefin and a thermoplastic phase based on propylene, wherein the elastomeric phase is at least 45% by weight relative to the total weight of the heterophase copolymer and this copolymer is substantially devoid of crystallinity deriving from polyethylene sequences.